Such mass flow meters have long been known. The measuring principle of thermal mass flow meters is based on the cooling of a heating element mounted on a holder when immersed into a flowing fluid. The flow which flows over the surface of the heating element absorbs heat from the latter and thus cools the heating element. The construction and behavior are illustrated in principle in FIG. 4. In this case, the quantity of heat absorbed by the flow depends on the temperature difference between the surface and the fluid, and on the flow itself. It can be described by a function{dot over (q)}=α(TO−TF),where{dot over (q)} is the quantity of heat dissipated,(TO−TF) is the temperature difference, andα is a constant of proportionality.
The constant of proportionality α is in this case directly dependent on the flow and is a function of the mass flow density over the heating element α=f(ρν)˜√{square root over (ρν)}. Now, if the temperature difference between the surface and the fluid, and also the heating power required to generate this temperature difference, are known, the mass flow over the heating element can thus be determined from this.
Therefore, for practical application of such a thermal mass flow measurement, two temperature sensors, one of which is heated and used for the flow measurement, are now put into the flow as illustrated in FIG. 5. The second temperature sensor serves to measure the fluid temperature TF.
In general, the measurement is in this case carried out only statically with a constant heating power or a constant temperature difference between the heater and the flow. However, a pulsed mode of operation, which is evaluated with slightly more effort, could also be carried out in this case.
The low directional sensitivity of the sensor is a peculiarity of this thermal mass flow measurement. The cooling effect of the flow on the sensor is determined to a first approximation by the magnitude of the flow velocity and not by its direction, so that the direction only has a small influence on the measured signal. The flow can thus be incident on the sensor from any direction. This only leads to a small change in the measured signal and to correspondingly small measuring errors. This has the advantage that the sensors do not react as sensitively to deviations from a perpendicular oncoming flow as are caused by installation tolerances, for example. Furthermore, for this reason, thermal flow sensors are also often constructed as symmetrically as possible in order to further reduce such measuring errors.
However, it is disadvantageous in this case that forward and reverse flows in a pipe cannot be distinguished by a simple thermal sensor. However, in the process, fluctuations in the flow are often caused by external influences and by the different components in the system, so that the flow does not always flow past the sensor in one direction, but backflows can also occur. This occurs in particular in the case of low flows. Since the sensor registers this flow independently of the flow direction, the backflow is also counted positively, which can lead to large erroneous measurements in the case of small flows and to the display of a flow in the case of zero flow. A direction detection of the flow is thus required to avoid such erroneous measurements.
The sensors are also in general calibrated in a preferred direction. They then nevertheless indicate approximately correct values in the case of being installed the wrong way round by 180°, so that the incorrect installation cannot be detected immediately due to a very unusual flow rate signal. The measuring errors resulting from the incorrect installation position are significantly higher than in the case of a correct installation position. In this case too, direction detection is advantageous to immediately generate a corresponding error message in the case of an incorrect installation position.
DE 33 04 710 A1 discloses for direction detection analyzing the behavior of the flow signal near the zero crossing in particular, and deriving a sign change from this. However, a prerequisite for this method is that information about the type of application in which the sensor is used is available in order to be able to form a corresponding model. For this reason, this can only be implemented with great difficulties in the case of a universal flow meter. The sensors are also not linked to another in many applications, so that additional information is not available. An incorrect installation can thus also not be detected in this way.
DE 34 17 051 C2, DE 102 18 117 B4, DE 31 35 794 A1 and DE 10 2004 039 543 A1 similarly use additional external information about the pulsation generator, in this case an internal combustion engine, in order to synchronize the flow measurement with the pulsation and thus avoid erroneous measurements. The disadvantages are comparable to those in the previously mentioned DE 33 04 710 A1.
EP 1 396 709 A1 thus discloses the arrangement of two flow-sensitive heating elements in a special housing in the flow in order to detect the flow direction independently of additional external information. This housing is designed such that, depending on the flow direction, the flow washes more strongly around one or the other heater and thus one or the other sensor is cooled more strongly.
This method allows independent flow direction detection. The additional housing and the additional sensor however imply a substantially increased effort for the production and operation of the sensor.
The proposition in accordance with EP 1 291 622 A2, in which only one flow-sensitive sensor is used, works in a similar vein. However, the sensor is located in a channel within a special sensor housing, with this channel having an inlet opening in the direction of the inlet flow, and an outlet which opens out to the side of the housing, so that a flow in the channel is caused by the ram pressure on the channel inlet and the sensor is thus only sensitive to flow in one direction.
In DE 10 2005 019 614 A1, a suitable housing also damps the pulsing flow to the sensor and partially suppresses it. However, the sensor for these methods is also quite complex, due to the required housing.
In contrast to the thermal mass flow meters described above, in which the cooling of the heater is used as the measuring effect, calorimetric thermal mass flow meters automatically also determine the flow direction, as shown in EP 1 310 775 A1, WO 2004/018976 A3 and EP 1 452 838 A2. In the case of calorimetric mass flow meters, in contrast to the to the principle described above, it is not the cooling of the heater that is measured, but rather there are two temperature sensors in the direct vicinity of the heater, one upstream and one downstream from the heater. In the case of a flow, the heat of the heater is transported by the flow to the downstream sensor and it registers a higher temperature. The flow velocity can then be determined from the temperature difference between the upstream sensor and the downstream sensor. If the flow direction of the flow changes, then the sign of the temperature difference correspondingly switches and the flow direction can be detected from this.
However, this measuring principle is limited to flows with a low Reynolds number, that is to say mainly laminar flows, since the heat in the flow is greatly distributed by the turbulence in the flow and the measuring effect is strongly reduced or even completely covered by the transport of heat in one direction. For this reason, only slow flows in narrow channels can be measured. In cases of higher fluid throughput, the sensitivity decreases and only corresponding by-pass solutions can be used here. These sensors are also generally produced with a low thermal mass in order to be able to react quickly and sensitively to the flow, so they are constructed in a correspondingly small and filigree manner. They are thus accordingly sensitive to external mechanical influences. For the field of application of large mass flow sensors described above, and also in severe environmental conditions, the are therefore unsuitable and can in general not be used as an alternative.